1. Field of the Invention
This invention is related in general to a level sensor which has high signal gain for fluids particularly chemically corrosive fluids and comprises a PTC-resistor formed with power leads that are attached to aluminum layers and which is encased in glass to form the sensor element.
2. Description of Related Art
The use of PTC-resistors for checking levels of fluid is known. A specific embodiment of a level sensor is described in publication Siemens-Zeitschrift, No. 2, February 1965, Pages 138-145.
PTC-resistors are temperature dependent electrical resistors (thermistors) made of semiconducting ferro-electric ceramic. It is characteristic of them that there is a steep rise of resistivity in the vicinity of the Curie temperature of the ferro-electric material. It is important for the application that the start of the nearly discontinuous rise of the resistance is several powers of 10 which is characterized by the reference temperature that approximately corresponds to the ferro-electric Curie temperature. At present, types of PTC-resistors which have reference temperatures to about +250.degree. C. are available. Below the Curie temperature, the PTC-resistors are low impedance.
The temperature of a PTC-resistor is defined first, by the temperature of its environment and second, by the Joule effect developed in the PTC-resistor itself. Which of these two contributions is dominant in the specific application depends on the electrical load and on the heat elimination conditions. When a voltage U is applied to the PTC-resistor and a current I flows through it, then a power P=U I exists in the resistor and the power is developed as heat. The temperature of the PTC-resistor T.sub.K then rises until a condition is reached where the same amount of heat is generated as is radiated and conducted to the environment. The PTC-resistor is then in thermal equilibrium with its environment. The relationship ##EQU1## is then valid wherein T.sub.U represents the ambient temperature and W is the thermal resistance.
When used as a level sensor, the PTC-resistor is operated with a fixed voltage which is adequate to heat it above the reference temperature. With a constant voltage, the current through the PTC-resistor is a measure of the existing conditions of thermal conduction. When the thermal resistance W changes then changes in power occur in the PTC-resistor and as a result, changes in the current occur which can be used to detect such changes. If the PTC-resistor is immersed into a fluid, the current power consumption rises because the PTC-resistor lowers its resistance due to the increased cooling effect of the fluid. Based on this result, level sensors can be designed which can signal level conditions such as "fluid present/not present".
The thermal resistance W is approximately additive and is composed of a part associated with the ambient temperature and a part associated with the PTC-resistor in its envelope. An optimum signal relationship is consequently obtained according to the above equation in the case of a low ambient temperature, a high reference temperature and good thermal coupling of the PTC-resistor to the environment. Unfortunately, the use of PTC-resistors having high reference temperatures is impossible in many applications. With increases of the reference temperature, the power consumption in the immersed condition also increases which leads to undesired current peaks. Also many ambient media are not chemically stable under high temperature conditions. At reference temperatures of 200.degree. C. the soldered connection between the power lead wires and the PTC-resistors will melt.
In the known level sensor wherein the PTC-resistor is integrated into a glass housing, wherein a layer of air is located between the PTC-resistor and the glass housing, an adequate signal relationship only results with a fluid temperature up to about 50.degree. C. Although level sensors for higher fluid temperatures are known from the prior art as, for example, from German Pat. No. 32 32 333, these are designed in a completely different manner and are constructed in a significantly more involved and complicated manner than the specially simple glass encapsulated level sensors of the invention.
In the known level sensor which is introduced into a glass housing, the two demands for tight integration of the PTC-resistor and good thermal coupling to the environment are essentially resolved separately from each other. The glass housing protects the PTC-resistor whereas the outward transfer of the heat occurs mainly by way of the power leads. A first power lead wire has a flat head into which the miniature PTC-resistor which is only about 1.3 mm wide and 0. mm thick is pressed using a spring clip which is secured to a second power lead wire. The PTC-resistor itself has a firmly adhering metal coat applied barrier free which in addition to causing the electrical contact also causes good thermal contact with the flat head of the power lead wire. As has been mentioned, however, practice has shown that a thermal coupling only by using the power lead wires does not assure an adequate signal relationship for ambient temperatures of more than 50.degree. C.
See also U.S. Pat. No. 4,276,536, U.S. Pat. No. 3,863,210, German No. 24 29 483, and European patent application no. 0,061,550.